Moisture control system with curl compensation

ABSTRACT

UPPER AND LOWER DRYING SECTIONS, EACH COMPRISING A PLURALITY OF STEAM-HEATED DRYING ROLLS, ARE EMPLOYED IN THE DRYING OF THE PAPER WEB AS IT PASSES THEREBETWEEN. A FIRST CONTROL LOOP IS ASSOCIATED WITH THE UPPER SECTION AND A SECOND CONTROL LOOP ASSOCITED WITH THE LOWER SECTION. THESE DRYER LOOPS DETERMINE THE RESPECTIVE AMOUNTS OF STEAM DELIVERED TO THE TWO SECTIONS. A MOISTURE CONTROL LOOP OUTPUTS A SET POINT TO THE FIRST DRYER LOOP AND A SECOND SET POINT IS PROVIDED FOR THE SECOND DRYER LOOP, THE SECOND SET POINT BEING MODIFIED SO THAT THE RATE OF STEAM FLOW TO THE SECOND DRYER SECTION IS MAINTAINED IN A PREFERRED RELATIONSHIP TO THE RATE SUPPLIED BY THE FIRST DRYER SECTION SO THAT THE CURL OF THE WEB IS MINIMIZED.

2 sheets-sheet 1 M. A. KEYES IV, ETAL 7. .QR kQ\ .|||||.I.|.||||!l QESEE $E sb wx fisnfimm E E x9383 w m E5 E6 \QQQQWEE Q F NPlb I!!! v v Q v4, v m6 h% Rh Y $3323 EQEESQ E5 in mgwwwg U @233 @EQg 9%. W, .Q.W/\ UN... by EQEQQS EQEQQE NR m. q $8 m S b Q sh wgwusq w m gwzwfi NM! Q #53 1 R QNM ESE MOISTURE CONTROL SYSTEM WITH CURL COMPENSATION 1 Feb. 23, 1971 Filed De 1a; 1968 wm w wm mq k NQQ L Kw W p QM $53 REE, X KUSQQMQEMQYU YKMQDQWZEQ QM United States Patent 01 3,564,724 MOISTURE CONTROL SYSTEM WITH CURL COMPENSATION Marion A. Keyes IV, South Beloit, Ill., and John Arthur Gudaz, Beloit, Wis., assignors to Beloit Corporation,

Beloit, Wis., a corporation of Wisconsin Filed Dec. 18, 1968, Ser. No. 784,665 Int. Cl. F26b 19/00 US. Cl. 34-48 13 Claims ABSTRACT OF THE DISCLOSURE Upper and lower drying sections, each comprising a plurality of steam-heated drying rolls, are employed in the drying of the paper web as it passes therebetween. A first control loop is associated with the upper section and a second control loop is associated with the lower section. These dryer loops determine the respective amounts of steam delivered to the two sections. A moisture control loop outputs a set point to the first dryer loop and a second set point is provided for the second dryer loop, the second set point being modified so that the rate of steam flow to the second dryer section is maintained in a preferred relationship to the rate supplied by the first dryer section so that the curl of the web is minimized.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates generally to paper making, and pertains more particularly to a control system for minimizing the amount of curl in the sheet by adjustment at a location just ahead of the reel on which the web is being wound.

(2) Description of the prior art It is generally known that curl or web curvature is caused by several factors. Orientation of the fibers throughout the web and internal stresses within the web are two factors. An important third factor, however, is the unequal distribution of moisture within the web itself. For example, if the web is drier adjacent its top surface than its bottom surface, there is a shinkage at the top that causes the web to curl upwardly. Compensation for curling, whether up or down, is necessary on paper grades, such as tab card stock, and any other situation where uniform sheet characteristics are critical.

In some prior art instances, the curl has been ignored and the paper has been of inferior quality. However, where the use of the paper demands that curl be cor rected for, it has proved exceedingly difficult to introduce a compensation that will assure uniformity in the final product. The manner of correction in the past has been largely left up to the judgment of the individual operator and his personal previous experience. In some instances, the operator has made a correction at one drying stage that proves excessive by the time the paper web reaches the reel end.

SUMMARY OF THE INVENTION One object of the invention is to provide a system for accurate compensation of the amount of curl. In this regard, it is an aim of the invention to make the needed compensation in a systematic manner so that uniformity is assured. More specifically, the present invention envisages the supplying of steam to upper and lower dryer sections at different flow rates so that one section contributes more heat than the other when the tendency to curl requires differing drying rates. In two embodiments, a ratio of the pressures that determine the drying rates is ice utilized and in two additional embodiments, the difference between the pressures is resorted to. The set points representing the ratios of pressures and the difference pressures, respectively, is obtained by initially observing the data and selecting the set points so that optimum results Will occur. When the necessary amount of correction has been realized, it is then known that the correct ratio or correct difference as far as set points are concerned have been entered by the operator and from that time on the ratio or difference will be automatically maintained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a typical paper making machine with the curl compensation obtained by means of a ratio loop in association with other control loops;

FIG. 2 depicts a modification of the invention, a difference control loop being substituted for the ratio control loop of FIG. 1;

FIG. 3 is another modification of the invention in which a multiplier circuit is substituted for the ratio control loop of FIG. 1, and

FIG. 4 is a fragmentary view illustrating still another modification that the invention may assume, the instant embodiment making use of subtraction rather than a ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT A conventional paper making machine has been indicated generally by the reference numeral 10. The machine 10 comprises the usual headbox 11 which distributes the pulp stock onto a Fourdrinier wire 14. The web 16 thus formed is directed through a wet press section 18 into a first dryer section 20. From the first dryer section 20 the web 16 proceeds to a size press 22 where an appropriate sizing additive is applied when such an agent is required. From the size press 22 the web 16 continues to a secondary dryer section 24 which includes what are sometimes referred to as upper and lower tiers but which will herein be referred to as upper and lower dryer sections, these actually being subsections of the dryer section 24. The upper section has been labeled 24a and is composed of any preferred number of steam-heated drying rolls, whereas the lower section has been designated by the numeral 2-412. These tiers or sections 2411 and 2412 are important to a practicing of the present invention as will soon become clear.

When calendering is desired, a calender stack is employed. Thereafter, a moisture gauge 32 senses the amount of moisture contained in the web 16 as the web is wound onto a take-up reel 34. The moisture gauge 32 may be of the scanning variety. Actually, the particular type of gauge is unimportant to an understanding of the invention.

A source or supply of steam for the various drying sections has been indicated by the reference numeral 36. A valve 38 is connected to the source of supply 36 and varies the amount or rate of flow of steam passing to the upper section 24a. A valve supplies steam to the lower section 24b. A current-to-pressure transducer 42 positions the valve 38 as a means to regulate the amount of steam delivered to the upper section 24a, whereas a similar transducer 44 positions the valve 40 as a means to regulate the amount of steam fed to the lower drying section 24b.

The system for controlling the transducers 42 and 44 has been generally designated by the reference numeral 50. As explained above, the moisture gauge 32 may be of the scanning type. Thus, the gauge 32 may be considered as traversing the web 16 and in this way obviates the probability of a control predicated on a very atypical point of the web 16. However, the system will operate on a single point as long as the single point is representative of the average moisture. However, assuming that a representative single point cannot be reliably obtained, a valid average computer 52 has been depicted having a first input line 52a connected to the moisture gauge 32 and a second input line 52b which receives a digital control signal only when the gauge 32 has completed a scan from front to back or vice versa.

The output line leading from the valid average computer has been labeled 52c and is connected to an input terminal 54a of a summing junction or comparator 54.

Hence, the terminal 54a introduces one input, this being the computed valid average moisture signal, into the summing junction 54. Of course, if a good moisture profile exists, the valid average need not be calculated and a single point would be sufiiciently representative of the moisture content of the web 16 so as to serve as the input to the summing junction 54.

The summing junction 54 has another input terminal 54b which carries the moisture set point which is representative of a desired moisture level that should exist in the web 16 just prior to the winding of the web onto the reel 34. The summing junction 54 has an output terminal 54c and any difference between the signal applied via the input terminal 54a and the moisture set point applied via the terminal 54b will appear as an error signal at the output terminal 540.

The output signal from the terminal 54c is fed to a moisture controller 56. The moisture controller then outputs a set point or a signal which functions as one of the inputs to a summing junction or comparator 58, being impressed upon one input terminal 58a belonging to this particular summing junction. Actually, it is the signal from the moisture controller 56 that constitutes a set point for a control loop for the upper drying section 24a. The set point derived from the moisture controller 56 is compared with a measured pressure signal coming from a pressure-to-current transducer labeled 59 which receives a pressure signal indicative of the actual steam pressure existing at the upper dryer section 24a.

Any difference between the incoming signals applied to the terminals 58a, 58b is outputed as an error signal on the terminal 580 which is connected directly to a pres sure controller 60. Thus, the pressure controller 60 operates on the error transmitted from the summing junction 58 and this error, which is a time function, determines to what extent the valve 38 should be repositioned, doing so through the ourrent-to-pressure transducer 42. In other words, the valve 38 is adjusted so as to vary the rate of steam flow to the upper dryer section 24a in accordance with the value of the error signal that is delivered to the pressure controller 60.

Mainly to enable a ready comparison of FIG. 1 with FIGS. 2-4 the moisture controller 56-, which outputs a signal used in the control of the upper dryer loop just described, is connected to circuitry generally denoted by the reference numeral 61 by a line 62' that extends thereto from the moisture controller 56. In the specific situation being described, the circuitry 61 constitutes a calculation ratio loop. To further facilitate a comparison with FIGS. 2-4 it will be observed that a line 63 is utilized leading into the circuitry 61 and a line 64 extending therefrom. The above-mentioned line 62 leads to an input terminal 65a belonging to a set point ratio calculator 65. It is the function of the set point ratio calculator 65 to produce a signal indicative of the ratio of two sets points, one set point being the set point provided by the moisture controller 56 and the other set point being the set point uti lized in a control loop for the lower dryer section 24b. It is via an input terminal 65b leading into the set point ratio calculator 65 that enables a comparison to be made of this particular set point with the set point applied to terminal 65a. The calculator 65 has an output terminal 65c.

The division performed by the calculator 65 results in the forwarding of a signal via the output terminal 650 which represents the ratio of the two set points applied to the input terminals a and 65b. The output terminal 65c is connected to an input terminal 66a belonging to a summing junction or comparator 66, the summing junction 66 having a second input terminal 66b which is connected to the previously mentioned line 63. In the present instance, it is the line 63 that carries a ratio set point which is a signal indicative of a desired ratio that should be maintained between the set points applied to the terminals 65a and 65b. Any difference between the signals impressed on the terminal 66a and 66b appears as an error signal on the output terminal 660. In this Way, a ratio controller 68 receives a signal that represents the difference between the inputs on the terminals 66a and 66b. The ratio controller 68 then provides a set point having a desired relationship with the set point derived from the moisture controller 56 which last-mentioned set point is utilized in a control loop for the lower dryer section 24b.

Included in the control loop for the lower dryer section 24b is a summing junction or comparator circuit 70 having a pair of input terminals 70a, 70b and an output terminal 70c. Whereas the input terminal 70a is connected to the ratio controller 68, the input terminal 70!) is connected to a pressure-to-current transducer 72 that forwards an electrical signal indicative of the actual steam pressure existing at the lower dryer section 24b. It is the difference between the inputs applied to the terminals 70, 1 and 70b that is differenced at the summing junction 70 to produce an error signal at the output terminal 700 which is fed to a pressure controller 74. Through the agency of the pressure controller 74 and the current-topressure transducer 44 the valve 40 is positioned to whatever extent is necessary to adjust the steam flow to the dryer section 24b.

Although the operation that occurs has been presented as the written description has progressed, nonetheless it will be of benefit, it is believed, to recapitulate briefly as to what transpires. Hence, a moisture control loop is provided by means of the moisture gauge 32, the computer 52, the summing junction 54 and the moisture controller 56. Two pressure control loops are provided, as hereinbefore alluded to. The first one which controls the upper dryer section 24a includes the transducer 59, the summing junction 58, the pressure controller 60 and the transducer 42 for adjusting the valve 38. The other pressure control loop includes the transducer 72, the summing junction 70, the pressure controller 74 and the transducer 44 for adjusting the valve 40. Set points are provided for each of these pressure control loops. The set point for the upper loop is derived directly from the moisture controller 56. On the other hand, the set point for the lower dryer section is dependent on the upper loop set point as influenced by the ratio set point that is determined by observing the resulting curl condition of the web 16 as it is wound onto the reel 34. If the initial value of the ratio set point that is adopted proves inadequate, another value is chosen. Once the ratio set point is properly determined, though, then the system 50 continues to produce a paper web 16 having a minimum degree of curl. Thus, the valve 40 will control the rate of steam flow to the lower section 24b in a predetermined relationship with the rate of steam flow to the upper dryer section 24a as determined by the position of the valve 38. In this way, one side of the web 16 can be dried at a different rate having preferred relationship with the other side, thereby producing a web having optimum curl characteristics imparted thereto.

Referring now to FIG. 2, it will be appreciated that the block 161 replaces the block labeled 61 in FIG. 1. In this embodiment, a differencing of the set points for the two pressure control loops is obtained. Thus, instead of a ratio control loop as set forth in FIG. 1, FIG. 2 deals with a difference control loop. Hence, the previously mentioned line 62 leads to a first input terminal 165a belonging to a differencing means or subtractor 165 which has a second input 165b. Thus, the subtractor 165 produces an output on the terminal 165a which represents the difference between the two dryer pressure set points. Included in the block 161 is a summing junction or comparator 166 having a first input terminal 166a connected to the output terminal 1650 and a second input terminal 166!) connected to the line 63. It is via the line 63 in the present instance that a difference set point is applied to the input terminal 166b and this is difterenced with the actual difference signal arriving via the input terminal 166a to provide an error signal representative of any disparity. The error signal is fed from the terminal 166c to a difference controller 168.

Hence, the set point provided by the difference controller 168 is fed back by means of the terminal 165b and is subtracted from the value of the set point impressed upon the input terminal 1650 which set point is derived from the moisture controller 56. Otherwise, the operation is the same as that provided by the system depicted in FIG. 1 labeled 50, the circuitry shown in block 161 being substituted for the circuitry of block 61 in FIG. 1.

Turning now to a brief description of FIG. 3, it will be perceived that this figure embodies a simplification of FIG. 1 as far as block 61 is concerned To orient the reader, a block labeled 261 is presented. However, the block 261 contains only a multiplier 265 having an input terminal 265a and an input terminal 26512 plus an output terminal 2650. The same set point provided by the moisture controller 56 arrives via the line 62 and is inputed at the terminal 265a. The same ratio set point utilized in FIG. 1 is applied to the input terminal 265a. The two set points are multiplied together to provide the second pressure set point for the lower control loop which includes the dryer section 24b. The operation is otherwise the same as that of the system shown in its entirety in FIG. 1.

The remaining modification is presented in FIG. 4. This arrangement may be considered as a simplification of FIG. 2. In this instance the replacement for the block 61 of FIG. 1 has been labeled 361. As with FIG. 2, the present modification operates on a difference between the set points rather than a ratio thereof. It will be discerned that the subtractor labeled 365 has an input terminal 365a and an input terminal 365b. It is the input terminal 365a that is connected to the line 62 which is in turn connected to the output sides of the moisture controller 56. Hence, the same set point utilized in FIGS. 1-3 is employed in the present situation. The set point representative of a desired difference between set point values is the same as that utilized in FIG. 2, the set point being applied via the line 63 and thus introduced to the subtractor 365 through the agency of its second input terminal 365b. The difference between the two inputed set points appears as an output on the terminal 365c which serves as the set point for the control loop associated with the lower dryer section 24b. In this instance, not only is a subtraction operation being performed in contradistinction to the ratioing of two set points, but it is done without feedback as resorted to in FIG. 2. Otherwise, the functioning of the modification succinctly presented in FIG. 4 is the same as the other situations herein described.

What is claimed is:

1. In apparatus for making a continuous web of paper comprising a first dryer section for directing heat against one side of said web, a second dryer section for directing heat against the other side of said web, first means for controlling the heat supplied to said first dryer section in accordance with the moisture contained in said web after said web has passed between said dryer sections, and second means responsive to said first control means for controlling said second dryer section in a preferred relationship to said first dryer section to reduce the curl of said web, said first control means including a comparator for comparing a signal representative of the actual moisture content of said web after said web has passed between said dryer sections with a set point representative of a desired moisture content of said web to provide an error signal which is a measure of any difference therebetween, said first means controlling heat supplied to said first dryer section in accordance with the value of said error signal and said second means being responsive to said error signal for controlling said second dryer section as a function of the value of said error signal.

2. The apparatus of claim 1 in which said error signal provides a set point for said first dryer section and said second means provides a set point for said second dryer section, the relative values of said last-mentioned set points determining said preferred relationship.

3. The apparatus of claim 2 in which said second means includes a ratio control loop having said first dryer section set point as one input and a ratio set point as a second input, said ratio loop causing adjustment of the value of said set point for said second dryer section to control said second dryer section in said predetermined relationship to said first dryer section.

4. The apparatus of claim 2 in which said second means includes a difference control loop having said first dryer set point as one input and a difference set point as a second input, said difference loop causing adjustment of the value of said set point for said second dryer section to control said second dryer section in said predetermined relationship to said first dryer section.

5. The apparatus of claim 2 in which said second means includes a ratio calculator having said first dryer section set point as one input and a ratio set point as a second input, said ratio calculator causing adjustment of the value of said set point for said second dryer section to control said second dryer section in said predetermined relationship to said first dryer section.

6. The apparatus of claim 2 in which said second means includes a difference calculator having said first dryer set point as one input and a difference set point as a second input, said difference calculator causing adjustment for said second dryer section to control said second dryer section in said predetermined relationship to said first dryer section.

7. In apparatus for making a continuous web of paper comprising a first steam-heated dryer section for directing heat against one side of said web, a second steam-heated dryer section for directing heat against the other side of said web, a first dryer control loop for adjusting the rate of steam flow to said first dryer section, a second dryer control loop for adjusting the rate of steam flow to said second dryer section, said first loop including means for comparing a signal representative of the steam pressure existing at said first dryer section with a first set point representative of the amount of moisture change required in said Web after passing between said dryer sections to provide a first error signal, a first valve for supplying steam to said first dryer section, means responsive to said first error signal for positioning said first valve to adjust the rate of steam flow to said first dryer section in accordance with the value of said first error signal, said second loop including means for comparing a signal representative of the steam pressure existing at said second dryer section with a second set point to provide a second error signal, a second valve for supplying steam to said second dryer section, means responsive to said second error signal for positioning said second valve to adjust the rate of steam fiow to said second dryer section, and means responsive to said first set point for changing the value of said second set point in a desired relationship as influenced by the value of said first error signal.

8. The apparatus of claim 7 in which said last-mentioned means includes a set point ratio calculator for providing a ratio signal representative of the ratio of said first set point to said second set point and a comparator for comparing said ratio signal with a ratio set point representative of a desired ratio to provide a third error signal, said second set point being determined by the value of said third error signal.

9. The apparatus of claim 7 in which said last-mentioned means includes a set point difference calculator for providing a difference signal representative of the difference between said first and second set points and a comparator for comparing said difference signal with a set point representative of a desired difference between said first and second set points to provide a third error signal representative of any difference therebetween, said third error signal determining the value of said second set point.

10. The apparatus of claim 7 in which said last-mentioned means subtracts a third set point from said first set point, said third set point being representative of a desired difference betweensaid first and second set points, the difference between said first and third set points determining the value of said second set point.

11. The apparatus of claim 7 in which said last-mentioned means multiplies said first set point by a third set point, said third set point being representative of a desired ratio between said first and second set points, to provide a product signal which product signal determines the value of said second set point.

12. In apparatus for making a continuous web of paper having first and second vertically arranged steam-heated dryer sections between which the web passes, a moisture control system for effecting curl compensation comprising a first dryer loop for controlling the flow of steam to said first section, a second dryer loop for controlling the flow of steam to said second section, a moisture loop for providing a first set point signal for controlling said first dryer loop in accordance with the difference between a signal representative of the actual moisture content of said web after passing between said rolls and a second set point signal representative of a desired moisture content of said web, and a ratio loop for dividing said first set point signal into a third set point signal for the second dryer loop and comparing the resulting signal with a fourth set point signal representative of the amount of needed curl compensation.

13. In apparatus for making a continuous Web of paper having upper and lower dryer sections between which the web passes, a first valve for supplying steam to said upper section and a second valve for supplying steam to said lower section, a moisture control system for effecting curl compensation comprising means for providing a signal representative of the actual moisture content of said web after passing between said sections, first comparator means for comparing said actual moisture signal with a desired moisture signal to provide a first error signal, means for providing a signal representative of the steam pressure at said upper section, second comparator means for comparing said first error signal with said pressure signal to provide a second error signal, means responsive to said second error signal for adjusting the position of said first valve so as to reduce the value of said first and second error signals, means providing a ratio signal representative of the ratio between said first error signal and a desired curl signal to provide a third error signal, means for providing a signal representative of steam pressure at said lower section, third comparator means for comparing said third error signal with said last-mentioned pressure signal to provide a fourth error signal, and means responsive to said fourth error signal for adjusting the position of said second valve so as to reduce the value of said third and fourth error signals.

References Cited UNITED STATES PATENTS 2,659,987 11/1953 Bennett 34-48 CARROLL B. DORITY, JR., Primary Examiner 

